The present invention relates to a developer composition for electrostatic image development in electrophotography, electrostatic recording, electrostatic printing and other fields.
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, the electrophotography art comprises a developing process in which an electric latent image is formed by evenly charging a photoconductive insulating layer, and subsequently exposing the layer to eliminate the charge in the exposed portion. The image visualized by adhering a colored, charged, fine powder known as a toner to the latent image, a transfer process in which the obtained visible image is transferred to a image-receiving sheet such as a transfer paper, and a fixing process in which the transferred image is permanently fixed by heating, pressure application or another appropriate means of fixing.
As stated above, a toner must meet the requirements not only of the development process but also of the transfer and fixing processes.
With respect to copying machines, printers, and other electrophotographic imaging apparatus, there have recently been strong demands for higher speed and better image quality. To meet this requirement, various methods and apparatus have been developed for toner image fixing. Among these methods, the most common is the heat roller fixing method, in which heat and pressure are applied at the same time.
The high speed copying machines and other apparatus based on this fixing method are liable to cause fixing failure due to a reduction in the heat roller temperature as a result of insufficient heat supply. This insufficient heat supply is due to a considerable loss of heat from the heat roller to the image-receiving sheet which occurs because fixing is continuously carried out in a large number of cycles. It is therefore desired to develop a toner which permits fixing at lower temperature for high speed copying machines and other apparatus.
A polyester resin is essentially superior in fixing performance; it is sufficiently fixable even by the non-contact fixing method, as described in U.S. Pat. No. 3,590,000, for example. However, when it is used with the heat roller fixing method described above, it is liable to cause the offset phenomenon and is thus difficult to use. In the offset phenomenon, a part of the molten toner, which comes into contact with the surface of the heat roller during heat roller fixing, transfers and adheres to the surface of the heat roller, which in turn again transfers to the image-receiving sheet supplied thereafter and stains the image.
With respect to the high speed copying machines and other apparatus based on the heat roller fixing method, there are strong demands for the development of the following toners:
A) A toner excellent in offset resistance. PA1 B) A toner excellent in low-temperature fixability.
Traditionally, various methods have been proposed for improving offset resistance, including:
1) the use of crosslinked resin (Japanese Examined Patent Publication Nos. 23354/1976 and 11902/1984 and Japanese Patent Laid-Open No. 90866/1984),
2) the introduction of a soft segment to a side chain to improve the low-temperature fixability while maintaining satisfactory offset resistance (Japanese Patent Laid-Open Nos. 109825/1982 and 7960/1984) and
3) the addition of low molecular weight polyolefin wax to provide the toner with detachability from the fixing roller (Japanese Patent Laid-Open Nos. 65232/1974 and 28840/1975).
However, in the method 1) described above, solely using a crosslinked resin increases the fixing temperature, and produces an unfixed portion under ordinary fixing conditions, resulting in image staining and other problems.
In the method 2) described above, although the fixability improves, the resin pulverizability, which is associated with toner productivity, becomes poor and the toner productivity is degraded because the resin toughness increases.
In the method 3) described above, addition of much low, molecular weight polyolefin wax leads to degradation of the charging property, due to a reduction in toner fluidity and other causes, though it is effective in the prevention of the offset phenomenon. In addition, no effect is obtained when the addition amount is small.